Using LINQ to Objects in C#

This article provides an introduction to employing LINQ to Objects queries to support a simple win forms application; the article addresses the construction of LINQ to Objects statements and then goes on to describe how one might use LINQ to Objects within the context of an actual application.

Introduction

This article provides an introduction to employing LINQ to Objects queries to support a simple win forms application; the article addresses the construction of LINQ to Objects statements and then goes on to describe how one might use LINQ to Objects within the context of an actual application.

The demonstration project included with the article is a simple contact manager which may be used to capture and store information about a person’s contacts in address book format. This demonstration application uses LINQ to Objects to manage, query, and order the list of contacts maintained by the application. The demonstration application also includes a dummy contact file with a collection of test contacts.

Figure 1: Application Main Form

The application provides the following functionality:

Create a contact data file.

Add contacts to the contact data file.

Remove contacts from the contact data file.

Search for specific contacts by last name.

Create and edit details about the contact.

First Name

Middle Name

Last Name

Street

City

State

Zip Code

Home Phone

Work Phone

Cell Phone

Email Address

Save a contact data file.

Reopen a contact data file.

Navigate through all of the contacts in the contact data file.

View a list of all contacts in the contact data file.

Provide a Rolodex function (search by starting letter of last name)

Naturally, the approaches used within the application are representative of only one way of doing things; as with most things in the .NET world, there are several alternatives and you can modify the code to work with the data using one of the other alternatives if you prefer to do so.

Figure 2: Searching for a Contact by Last Name

Figure 3: Listing All Contacts(Edits to the grid are posted immediately to the List)

Figure 4: Rolodex Function

LINQ to Objects Statements

This section will discuss some of the common techniques used in LINQ to Objects statement construction. In a nutshell, LINQ to Objects provides the developer with the means to conduct queries against an in-memory collection of objects. The techniques used to query against such collections of objects are similar to but simpler than the approaches used to conduct queries against a relational database using SQL statements.

In the example, an array of strings (tools) is used as the collections objects to be queries using LINQ to Objects; the LINQ to

Objects query is:
var list = from t in tools
select t;

In this example, an untyped variable “list” is created and all of the items contained in the string array are added to this object; the types are inferred (implicitly typed), for example, “t” is a member of tools, since it is known that tools is a string array, the framework will infer that “t” is also a string. Of course this is not all that terrific since you can just iterate through the array to do essentially the same thing; however, you can create more complex queries with LINQ to Objects and then the value of the LINQ library becomes more apparent.

If you were to create a project, add this bit of code to a method and run it, the results would look like this:

Figure 5: Query Results

Example 2 – Select with a Where Clause

The next example shows a LINQ to Objects query that incorporates a where clause. In this example, we start out with a collection of birds in the form of a string array; LINQ to Objects is used to query this string array to find and return a subset of the array in the form of all birds with names beginning with the letter “R”.

Example 6 – Searching a List<T> Using LINQ to Objects

In this example, a typed list is created (as in the previous example), populated, and then queried using LINQ to Objects. In this case, the query includes a where clause that only returns matches were the part description begins with the letter “S”:

The preceding examples were intended to provide a simple overview as to how to conduct some basic queries against collections using LINQ to Objects; there are certainly a great number of more complex operations that can be executed using similar procedures (grouping, joins and selects into a new custom type, etc.).

Getting Started

There is a single solution included with this download, the solution contains a Win Forms project called “LinqToObjects”; this project contains two forms (the main form (frmContactBook) and a form used to display the total list of contacts (frmFullList), a serializable class called Contact (used to contain contact related data), and a class entitled, Serializer which contains two static methods used to serialize and deserialize the contact data (writing it to and reading it from a file) .

If you open the attached project into Visual Studio 2008; you should see the following in the solution explorer:

Figure 12: Solution Explorer

Code: Contact.cs

The Contact class is the container class used to store all of the contact related data used in the application. Whilst this demonstration uses contact data, this could easily be replaced with something more useful to you.

The class begins with the normal and default imports:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

The next section contains the namespace and class declarations. Note that the class is declared as serializable; the serializable attribute indicates that the class can be serialized.

namespace LinqToObjects
{
[Serializable]
publicclass Contact
{

The region defined in the class declares the member variables used internally by the class; any member variables exposed externally are made accessible through public properties.

The next region of code in the class contains the constructors. Two constructors are defined; a default constructor that creates a new instance of the class and assigns it an internal ID (as a Guid). The second constructor accepts an ID as an argument and sets the contact’s internal ID to that value.

The last bit of the code in this class is contained within the properties region; this region contains all of the properties defined to access the member variables. Note that since the ID value is always set by the constructor, the property does not provide a public interface to set the Guid to a new value.

Code: Main Application Form (frmContactBook.cs)

The is the main form of the application; much of the code provides the framework for the application and does not really pertain to LINQ to Objects, however, all of the code will be described herein to provide a proper context. The contact application’s main form contains the following controls:

Menu

File

New

Open

Save

Save As

Exit

Contacts

Add Contact

Remove Contact

List All Contacts

Toolbar

Add

Remove

Find by Last Name

Save Data

Navigate to Previous Contact

Navigate to Next Bird Contact

Exit Application

Split Container Left Hand Side

Alphabet List

Alphabetized Names List

Split Container Right Hand Side

First name text box control

Middle name text box control

Last name text box control

Street text box control

City text box control

State text box control

Zip code text box control

Home phone number text box control

Work phone number text box control

Cell number text box control

Fax number text box control

Email address text box control

Figure 13: frmContactBook.cs

The class begins with the normal and default imports:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

The next section contains the namespace and class declarations.

namespace LinqToObjects
{
publicpartialclass frmContactBook : Form
{

The region defined in the class declares the member variables used internally by the class; any member variables exposed externally are made accessible through public properties. The comment adjacent to each declaration describes its purpose.

The next region of code in the class contains the constructor. Upon initialization, the application creates a new contact data list, creates a new contact data object, sets the current position indicator to zero, and sets the dirty form Boolean to false.

The next code region is called ‘Toolstrip Event Handlers’; the first event handler in this region is the click event handler for the Add button; this method merely calls the menu control’s click event handler and the code contained in that event handler adds a new contact to the current contact data.

The next handler is used to search for a specific contact using the contact’s last name. The code uses a LINQ to Objects query in order to find the first instance of a matching contact with that last name. The handler uses the search term text box control on the toolstrip to capture the last name and it uses the search button to execute the search. The code is annotated to describe what is going on in this method.

///<spanclass="code-SummaryComment"><summary></span>/// Find a specific contact by the contact's
/// last name
///<spanclass="code-SummaryComment"></summary></span>///<spanclass="code-SummaryComment"><paramname="sender"></param></span>///<spanclass="code-SummaryComment"><paramname="e"></param></span>privatevoid tsbFindContact_Click(object sender, EventArgs e)
{
// return if the search term was not provided
if (String.IsNullOrEmpty(tspSearchTerm.Text))
{
MessageBox.Show("Enter a last name in the space proved.", "Missing
Search Term");
return;
}
try
{
// using linq to objects query to get first matching name
var foundGuy =
(from contact in contacts
where contact.LastName == tspSearchTerm.Text
select contact).FirstOrDefault<Contact>();
// set the current contact to the found contact
currentContact = foundGuy;
currentPosition = contacts.IndexOf(currentContact);
// update the display by loading the
// found contact
LoadCurrentContact();
// clear the search term textbox and return
tspSearchTerm.Text = string.Empty;
return;
}
catch
{
MessageBox.Show("No matches were found", "Search Complete");
}
}

The next handler is used to navigate back one contact from the current position of the displayed contact. If the contact as at the lower limit; the button click is ignored.

///<spanclass="code-SummaryComment"><summary></span>/// Navigate back to the previous record
/// if not at the lower limit
///<spanclass="code-SummaryComment"></summary></span>///<spanclass="code-SummaryComment"><paramname="sender"></param></span>///<spanclass="code-SummaryComment"><paramname="e"></param></span>privatevoid tsbNavBack_Click(object sender, EventArgs e)
{
// capture form changes and plug them
// into the current contact before
// navigating off the contact
SaveCurrentContact();
// don't exceed the left limit
if (currentPosition != 0)
{
currentPosition--;
currentContact = contacts[currentPosition];
LoadCurrentContact();
}
}

The next handler is used to navigate forward one contact from the current position of the displayed contact. If the contact as at the upper limit; the button click is ignored.

///<spanclass="code-SummaryComment"><summary></span>/// Navigate to the next record if not at the
/// upper limit
///<spanclass="code-SummaryComment"></summary></span>///<spanclass="code-SummaryComment"><paramname="sender"></param></span>///<spanclass="code-SummaryComment"><paramname="e"></param></span>privatevoid tsbNavForward_Click(object sender, EventArgs e)
{
// capture form changes and plug them
// into the current contact before
// navigating off the contact
SaveCurrentContact();
// don't exceed the right limit
if (currentPosition < contacts.Count - 1)
{
currentPosition++;
currentContact = contacts[currentPosition];
LoadCurrentContact();
}
}
#endregion

The next region contains the menu item click event handlers. The first menu item is used to add a new contact to the current contact list. This method calls the function SaveCurrentContact which saves any entries currently made to the form to the current contact object; it then creates a new instance of a contact, adds the new contact to the contact list, clears the form, and marks the form as dirty since a new contact was added thus changing the list.

The next menu item click event handler creates a new contact list; before following through with the creation of the new contact list, this handler checks to see if the current form is dirty to allow the user the opportunity to save before closing the current list. Following that, the contact list is replaced with a new contact list and the form’s controls are cleared.

///<spanclass="code-SummaryComment"><summary></span>/// Create a new contact file but check for
/// a dirty form first and allow the user to save
/// if the current data has changed but not
/// been saved.
///<spanclass="code-SummaryComment"></summary></span>///<spanclass="code-SummaryComment"><paramname="sender"></param></span>///<spanclass="code-SummaryComment"><paramname="e"></param></span>privatevoid newToolStripMenuItem_Click(object sender, EventArgs e)
{
if (dirtyForm == true)
{
if (MessageBox.Show(this, "You have not saved the current contact
data; would you like to save before starting a new " +
"contact database?", "Save Current Data",
MessageBoxButtons.YesNo) ==
System.Windows.Forms.DialogResult.Yes)
{
saveAsMenuItem_Click(this, new EventArgs());
}
else
{
// discard and start new document
contacts = new List<Contact>();
ClearScreen();
}
}
else
{
// start new document
contacts = new List<Contact>();
ClearScreen();
}
}

The next event handler is used to open a contacts file. Again, the handler checks to a dirty form and provides the user with an opportunity to save if the form is dirty. A separate open method is called to handle the actually file opening operation.

///<spanclass="code-SummaryComment"><summary></span>/// Open an existing contact data file but
/// first check for a dirty form and allow the
/// user the opportunity to save it before
/// leaving the current contact file
///<spanclass="code-SummaryComment"></summary></span>///<spanclass="code-SummaryComment"><paramname="sender"></param></span>///<spanclass="code-SummaryComment"><paramname="e"></param></span>privatevoid openToolStripMenuItem_Click(object sender, EventArgs e)
{
if (dirtyForm == true)
{
if (MessageBox.Show(this, "You have not saved the current contact
data; would you like to save before opening a different " +
"contact database?", "Save Current Data",
MessageBoxButtons.YesNo) ==
System.Windows.Forms.DialogResult.Yes)
{
saveAsMenuItem_Click(this, new EventArgs());
}
else
{
Open();
}
}
else
{
Open();
}
}

The next handler exits the application after again checking to see if the form is dirty.

///<spanclass="code-SummaryComment"><summary></span>/// Exit the application but first check for
/// a dirty form and allow the user to save the file
/// before leaving the application
///<spanclass="code-SummaryComment"></summary></span>///<spanclass="code-SummaryComment"><paramname="sender"></param></span>///<spanclass="code-SummaryComment"><paramname="e"></param></span>privatevoid exitToolStripMenuItem_Click(object sender, EventArgs e)
{
if (dirtyForm == true)
{
if (MessageBox.Show(this, "You have not saved the current contact
data; would you like to save before exiting?", "Save Current
Data",
MessageBoxButtons.YesNo)==System.Windows.Forms.DialogResult.Yes)
{
tsbSave_Click(this, new EventArgs());
}
else
{
Application.Exit();
}
}
else
{
Application.Exit();
}
}

The save menu item is used to save the current contacts file to disk; the function first calls a SaveCurrentContact which is used to save the current contact to the current contact data list. Next, the function uses the save file dialog to capture a file name if none is currently set to the currentFilePath variable, or, if the variable is set, it saves the file using that file path. The file is actually saved to disk when the call to serialize the file is made.

The next bit of code us used to support the “Save As” menu item; the call is similar to the previous save method but straight opens the Save File dialog box to permit the user to name or rename the file.

The next method removes the current contact from the contact list and updates the position of the current contact.

///<spanclass="code-SummaryComment"><summary></span>/// Delete the current contact and update
/// the display
///<spanclass="code-SummaryComment"></summary></span>///<spanclass="code-SummaryComment"><paramname="sender"></param></span>///<spanclass="code-SummaryComment"><paramname="e"></param></span>privatevoid removeToolStripMenuItem_Click(object sender, EventArgs e)
{
// make sure there are records
if (contacts.Count == 0)
{
// remove the current record
contacts.Remove(currentContact);
// check to see if the current
// position is at the limit
// and move up or down
// as required
if (currentPosition == 0)
currentPosition++;
else
currentPosition--;
// reload the current contact
// from the new position
currentContact = contacts[currentPosition];
LoadCurrentContact();
// dirty the form since a
// record was removed
dirtyForm = true;
}
}

The next method is used to alphabetize the contacts and pass the resulting list to a new instance of the form used to display all of the contacts in a data grid view control. The current contact list is alphabetized by last name, first name, and middle name.

///<spanclass="code-SummaryComment"><summary></span>/// Create an ordered list of contacts and display that list
/// in a datagrid
///<spanclass="code-SummaryComment"></summary></span>///<spanclass="code-SummaryComment"><paramname="sender"></param></span>///<spanclass="code-SummaryComment"><paramname="e"></param></span>privatevoid listAllContactsToolStripMenuItem_Click(object sender,
EventArgs e)
{
// use linq to objects to create a list of contacts
// ordered by the contact's last name, first name,
// and middle name
var orderedCons =
(from contact in contacts
orderby contact.LastName ascending,
contact.FirstName ascending,
contact.MiddleName ascending
select contact);
// create an instance of the full list form and pass it's
// constructor the list converted to a List<Contact>
frmFullList f = new frmFullList(orderedCons.ToList<Contact>());
f.Show();
}
#endregion

The next region contains a garbage can collection of other methods maintained in a region entitled “Housekeeping”:

#region Housekeeping

The first method contained in this section is used to clear all of the text boxes used to display contact information. This is called anytime the current contact is changed to prevent remnants of one contact appearing the display of a replacement contact.

The next method captures all of the information currently on the form for the current contact and writes it into the current contact’s properties. This is called whenever a contact is changed so that all edits to an existing contact are held within the local list until it can be written to disk. The method further updates the order of the contacts and updates the contact list and displayed contact.

The final region in this form class is used to handle the listbox control events. These controls are used to provide a Rolodex sort of functionality to the application. The listbox controls are loaded into the left hand split panel’s panel. The top listbox control displays all of the letters in the alphabet whilst the lower listbox control is used to display all matching last names beginning with the letter selected in the upper listbox.

#region Listbox Event Handlers

The first function handles the selected index changed event for the upper listbox containing all of the letters of the alphabet. When a new letter is selected, this method uses a simple LINQ to Objects query to find all contacts with last names beginning with the selected letter. The lower listbox is then cleared and then the matches are then formatted into a string showing the contact’s last name, first name, and middle name and each formatted string is then added to the lower listbox control.

The the names listbox selected index changed event is handled in the next block of code. In it, the name string (Last name, first name, middle name) is parsed and used in a LINQ to Objects query used to return a list of all matching names; the first found name is displayed in the contact form and the index position is updated to support the list navigation.

///<spanclass="code-SummaryComment"><summary></span>/// Find the matching contact for the name picked
/// from this list box and display that contact's
/// information in the contact display area
///<spanclass="code-SummaryComment"></summary></span>///<spanclass="code-SummaryComment"><paramname="sender"></param></span>///<spanclass="code-SummaryComment"><paramname="e"></param></span>privatevoid lstNames_SelectedIndexChanged(object sender, EventArgs e)
{
// if there were no matches found, return from this function
if (lstNames.SelectedItem.ToString().Trim() == "No matches were found")
return;
// variables to hold parts of the name as search terms
string first, middle, last;
// get the last name
string[] arr = lstNames.SelectedItem.ToString().Trim().Split(',');
last = arr[0].Trim();
// get the first name
string[] arr2 = arr[1].ToString().Trim().Split(' ');
first = arr2[0].Trim();
// get the middle name
middle = arr2[1].Trim();
// cannot complete the query without the three values
// so return if the information is missing
if (String.IsNullOrEmpty(last) ||
String.IsNullOrEmpty(first) ||
String.IsNullOrEmpty(middle))
{
MessageBox.Show("This query requires a first, middle, and a last
name.",
"Missing Name Values");
return;
}
try
{
// using linq to objects query to get a collection of matching names
// when all three names match
var foundGuy =
(from contact in contacts
where contact.FirstName.Equals(first) &&
contact.LastName.Equals(last) &&
contact.MiddleName.Equals(middle)
select contact).FirstOrDefault<Contact>();
// set the current contact to the first found
// contact
currentContact = foundGuy;
// update the index position used to maintain
// the current position within the list
currentPosition = contacts.IndexOf(currentContact);
// reload the current contact and return
LoadCurrentContact();
return;
}
catch (Exception ex)
{
MessageBox.Show(ex.Message, "Error Encountered");
}
}
#endregion

Code: frmFullList.cs

This form class contains a data grid view control and a constructor which accepts a contact list (List<Contact>) as an argument. Upon initialization the list is bound to the data grid view control. Changes made by edits in the grid are maintained in the contact list.

Summary

The article shows some simple examples of LINQ to Objects queries used in support of a sample application. LINQ to Objects may be used to generate more complex queries than are shown in the example, however, those demonstrated herein are representative of some of the more common tasks that one might choose to do within a similar application. Much of the code in the demonstration project was provided as a framework for the application and was necessary to create an environment useful for testing some simple LINQ to Objects based queries.

I would also introduce this just to show people how some powerful functions work in LINQ in ways most people would not think about. Of course this does not make the code better, it's just cool showing what C#3.0 and LINQ can do

Change "\r\n" to Environment.Newline (I just did that so the line would fit)

Now keep in mind both your code and my code is not designed for large data sets. For example, the + operator should not be used inside of Append.

Again it's important for developers to remember there is a time to optimize and a time to be elegant (while remaining understandable). This last part is not something I would put into production code but it's a great demonstration for learning.